1. Field of the Invention
The present invention relates to a DC power source and more specifically to a DC power source including insulation type converters and a plurality of cells.
2. Description of the Related Art
Recently, the interest in fuel cells, which generate electricity via an electrochemical reaction between hydrogen and oxygen, as a source of power that operates efficiently and is environmentally friendly, has increased. Fuel cells are able to output a desired amount of power by controlling an amount of fuel gas supply. However, the output power response may be delayed due to a response delay in the gas supply.
As a means for addressing this disadvantage, technology has been suggested for configuring a power source by connecting a fuel cell and a secondary cell in parallel. For example, in the technology described in Japanese Patent No. JP 2000-12059, a combined use of a secondary cell and fuel cell is attempted by converting an output voltage of a fuel cell with a DC-DC converter.
In such an arrangement, further improvement in power output efficiency is required, as described in Japanese Patent No. JP 2002-118981. This reference describes “a DC power source comprising: a pair of terminals for outputting power, a fuel cell connected to the terminals, a secondary cell capable of charge and discharge, and a DC-DC converter, in which the secondary cell is connected to the terminals through the DC-DC converter in parallel with the fuel cell.”
According to JP 2002-118981, “usually, power loss occurs in voltage conversions in a DC-DC converter. In the present invention, a DC-DC converter is provided to the side of a secondary cell, so power loss of output from a fuel cell can be prevented. In a power source with a combined use of fuel cell and secondary cell, a case that power is supplied mainly from a fuel cell is usual. Thus, by preventing loss at a fuel cell of a frequent use, output efficiency of entire power source can be improved. In the view that use of a fuel cell is frequent, the present invention is especially effective in the case a maximum output of a fuel cell is larger than a maximum output of a secondary cell”.
However, in the system disclosed in JP 2002-118981, a DC-DC converter is connected to a pair of terminals in parallel (See FIG. 1 of JP 2002-188981). Therefore, output voltage of the DC-DC converter increases, and output power of the DC-DC converter also increases in a DC power source. Thus, an amount of power loss, e.g., lost as heat and so forth, increases in converting power of a secondary cell at the DC-DC converter.
The DC power source according to JP 2002-118981 will be further described below. FIG. 7 is a schematic view of a configuration of a conventional DC power source connected to a motor through an inverter. FIG. 7 provides a simplified view of FIG. 1 in JP 2002-118981. In FIG. 7, a DC power source 100 is connected to a secondary cell 12 through a DC-DC converter 13, with a fuel cell 11 connected to a pair of terminals 100a and 100b. A motor 15 constituting a part of a “drive unit” is connected to the DC power source 100 through an inverter 14.
The DC-DC converter 13 in the DC-power source 100 is desirably 100% efficient (although this is not practicable). The DC power source 100 provides, for example, a voltage of 48V and a current of 80 A to the inverter 14, and approximately 4 kW of power produced by this is inverted and output to the motor 15. To make this possible, the fuel cell 11 needs to produce approximately 1 kW power with a voltage of 48V and current of 20 A, and the secondary cell needs to produce approximately 3 kW power with a voltage of 36V and current of 85 A. The voltage is converted by the DC-DC converter 13 so that the voltage, current and power of the secondary cell 12 are output to the DC-DC converter 13 as electric power of approximately 3 kW with a voltage of 48V and current of 60 A.
However, the efficiency of the DC-DC converter 13 cannot be 100%. For example, if the efficiency of the DC-DC converter 13 is 80%, the necessary power supply for the DC-DC converter 13 to output 3 kW is 3.75 kW, and approximately 750 w becomes power loss. Therefore, the output of 3.75 kW is necessary to cover the power loss at the DC-DC converter 13 by the output voltage of the secondary cell 12. Thus, it is desirable to reduce as much as possible the power loss (e.g., 750 w) at the DC-DC converter 13 so that the motor 15 can be driven efficiently.